Various phases of research in thyroid function and iodine metabolism are being pursued as follows: 1. Purification of thyroid peroxidase and determination of its chemical and physical characteristics. 2. Determination of the role of thyroid peroxidase in iodination and in thyroxine biosynthesis. 3. Elucidation of the mechanism of iodination of thyroglobulin and of the mechanism of iodothyronine biosynthesis. 4. Elucidation of the mechanism of action of the clinically used antithyroid drugs. 5. Studies of the changes in iodine metabolism and in thyroid function that accompany mild and severe iodine deficiency. BIBLIOGRAPHIC REFERENCES: "The Mechanism of Inhibition of Thyroid Peroxidase-Catalyzed Iodination by Prophlthiouracil and Methylmercaptiomidazole" by Alvin Taurog. In Press Proceedings of the 7th International Thyroid Conference, Boston, Mass., June 9-13, 1975. "Acute and Chronic Responses to Iodine Deficiency in Rats" by G. Riesco, A. Taurog, P.R. Larsen and L. Krulich. In Press Proceedings of the 7th International Conference, Boston, Mass., June 9-13, 1975.